doublegunshop.com - home Forums General Discussion DoubleGun BBS @ doublegunshop.com Barrel wall thickness - safe?

"IF" one is Concerned about a set of barrels with good wall thickness at the breech end but thin at 18+ inches from the breech, shoot the load which gives the highest breech pressure commensurate with the overall desired ballistics.
You may for instance shoot two 1 oz loads which both reach 1200 fps muzzle velocity. If one of those loads has 3K "Less" "MAX Chamber Pressure" than the other, it's going to make it up some where else. I TRULY WONDER just where that might Be.

A note on proof, "hillbilly" or any other. Proof firing is necessary to get to after proof viewing. The test is only as good as the viewer and the viewing. Guns most usually don't disappear in a blinding flash and a resounding thunderclap. But, sometimes parts deform beyond their elastic limit. It is the duty of the viewer to "see" this deformation - via eyeballs or gages. The "hillbilly" method usually skips over the viewing. I'd bet sporting money that Dennis (above) gives every gun he proofs a really good viewing.

DDA

.016" is the thickness of the cover on a book of matches, or about the thickness of 5 average sheets of newspaper. To me that's not very comforting for a gun barrel. It may hold the pressure, but there is not much for margin of safety nor much strength to resist denting. There are too many good guns with adequate barrel thickness to risk shooting junk. Some people also get away for a long time driving on bald tires... until they don't.

This would be a good time for Doc Drew to put up that picture of the guy holding the blown-up shotgun with his hand missing a finger.

Miller, can you show us test graphs or point to references showing pressure rises from chamber to muzzle that illustrate your point regarding your statement about identical muzzle speeds and loads, lower pressures, etc. ? Thanks. Gil

Here you go Gil. "Smokeless Shotgun Powders: Their Development, Composition and Ballistic Characteristics" by Wallace H Coxe, 1931.

Ballistite is an early Dense Smokeless - higher chamber pressure
Schultze and DuPont Bulk Smokeless
DuPont Oval an early progressive burning powder - lower chamber but slightly higher at 4"



DuPont MX Smokeless was a Dense Multi Base Powder introduced about 1930. 25.5 grains was a 3 Dram Eq. and it was promoted as a 1 1/4 oz. Trap load. It was replaced in the 1954 with IMR PB.

This shows how pressure is higher at 18" with Dense vs. Bulk Smokeless

32 Gr Ballistite (1 1/8 oz. at 1232 fps = 3 Dram Equiv.)
Breech - 2.9 Tons per Square Inch (UK, Long) = 8624 psi
(2.9 x 1.5 = 4.35, 4.35 - .5 = 3.85 TSI, 3.85 x 2240 = 8624 psi)
2 1/2" - 2.2 Tons = 6272 psi
6� - 1.2 Tons = 3718 psi
12� - .81 Tons = 1602 psi
18� - .33 Tons
24� - .21 Tons

42 Gr Schultze 1 1/8 oz. at 1220
Breech - 1.69 Tons = 4558 psi
2 1/2� - 1.49 Tons = 3886 psi
6� - 1.36 Tons = 3450 psi
12� - .92 Tons = 1971 psi
18� - .55 Tons = 1178 psi
24� - .32 Tons

This is a pressure - time curve from Neil Winston - www.claytargettesting.com. The shot leaves the (30") barrel at about 3.5 milliseconds after ignition. The pressure is down to about 500 psi by 2 milliseconds.



More here
More here
https://docs.google.com/document/d/1F2sQuPm05IE4VWYYnCkvuXmYEzQoWd_SQgaAfUOZEFU/preview

Part of the discussion should clearly state where the thin spots are in relation to the forward hand. People pay extra for target barrels that have holes in them!

Posted here also in case the thread comes up in a future search. Note some barrels clearly are eccentric.



MEASURING WALL THICKNESS PROPERLY requires the expertise, time, and interest to do so.
One pass on one surface of the barrel is not adequate.

The 16 gauge has 28" barrels, the .016 thin spot is 12" from muzzle, balance is .023.

As the carpet dealer asks for a ruler, the assistant replies, "Which one? The one for selling or buying"

Thank you for the informed opinions

Note in the chart which Drew posted how the curves all cross between 3"-4" at which point the low pressure becomes the high pressure load & Vice-Versa. "IF" you push the same shot load to the same velocity you have done the same amount of work. That work in a shotgun barrel is accomplished by pressure. Thus if you lower it at one point you raise it somewhere else.
Now that said the "Peak" pressure occurs for a very short distance & extremely short time while the raised portion is spread out over virtually the rest of the barrel. in either case the pressure down the barrel is much lower than the max/peak pressure which occurs in the chamber.
Bottom line though is if you keep the same load & just change powders to drop the peak pressure you have not made it a bit easier on the gun other than directly in the chamber. These low pressure loads are actually most beneficial if you have a gun with very light breeches having thin walls over the chamber & particularly the junction of chamber & cone. In most guns this is not the case, particularly those built after the introduction of smokeless when this area was generally Beefed Up.

Get out a dial caliper and set it on .016 take a long look and If you're still brave and don't care about keeping all your fingers I say shoot away....